The present invention relates to a key input device for use in a pocket-type electronic calculator that utilizes a single LSI chip for both arithmetic and operational functions.
In recent years, various types of electronic circuits have been fabricated into integrated circuits. For example, the arithmetic and operational circuits for electronic calculators are now frequently incorporated on a single LSI chip. With this development, the electronic calculator has remarkably been reduced in size to be handy and compact. The miniaturization has been accompanied by consumer demand for multiple functions, which requires an increased number of key switches. Any increase in the number of key switches leads to a complementary increase in the number of signals inputted to the operational circuit. A drawback of using a single chip for both circuit control and arithmetic processing is that the chip has a limited number of pins (terminals) which may be connected to the peripheral circuits. The necessary peripheral circuits include such features as the key-input device, the display panel and a power source. Thus, there have been numerous attempts to accommodate the increased number of inputs to the operational circuit.
Merely increasing the number of the key switches results in an increase in the number of terminals. This complicates the wiring of the circuit, which is undesirable from a circuit design standpoint. One approach is to design the key switch to have two or more functions, that is, use of the multifunction keys, to overcome the above problem. This approach, however, makes the key operation quite complicated. Another approach is to use common terminals for both the display and alphanumerical key terminals for time-divisional use. This approach also makes a problem in that an insufficient level for keying in data may occur and a variation in the threshold voltage of the judging circuit for the key-in signal may cause an erroneous data entry.